An electrostatic precipitator generally comprises an array of vertically extending dust-collecting electrodes which are maintained at a relatively positive polarity by being connected to one terminal of a high-voltage direct-current source, an array of discharge electrodes having points (thorns or barbs) or sharp edges constituting corona-discharge regions. Below the base of the collecting electrodes, suitable collecting bins are provided and the collecting electrodes are associated with rapping means for dislodging the collected dust into the bins. The dust-collecting assembly thus consists of a pair of such dust-collecting electrodes defining a passage for the gas between them, the collecting electrodes spacedly flanking the array of corona-discharge electrodes.
Generally speaking, the corona-discharge electrodes are at negative polarity and are connected to the opposite terminal of the high-voltage direct-current source. The corona discharge generated between the edges or barbs (points) of the discharge electrode and the opposite portion of a collecting electrode results in ionization of gases and the generation of charged particles, e.g. negative ions or electrons which are picked up by the dust particles so that the latter become negatively charged. The dust particles are then attracted to and collect upon the positively charged collector electrodes to which they adhere electrostatically or mechanically until the collecting electrode is rapped to dislodge the dust into the bins.
It has been found to be advantageous to pass the gas which can be laden with the dust generally horizontally between the vertical collecting electrodes.
As will be apparent hereinafter from the specific description of both the prior art and the invention, the process just described, in the case of high-resistance dusts, results in some reverse ionization at the side of the collecting electrode at which the dust accumulates. As a result, positively charged dust particles may be released or formed by such reverse ionization and naturally such positively charged particles are repelled from and not attracted to the positively charged dust-collecting surface. As the gas stream passes horizontally between the dust-collecting electrodes, therefore, particles which pick up a positive charge by reverse ionization proximal to a collecting electrode tend to move toward the next discharge electrode at which they may pick up a negative charge and then move toward the collecting electrode where they may again pick up a positive charge, etc. Viewed as a statistical phenomenon, therefore, particles of dust tend to move in a zig-zag fashion between the plane of the discharge electrodes and the collecting electrodes spaced therefrom as the gas entrains such particles along the collecting path. The zig-zag movement is a phenomenon which is associated with high-resistance dusts.
Because of the zig-zag phenomenon, the effectiveness of dust collection is obviously reduced and hence the performance of a dust-collecting or dust-arresting assembly will be substantially lower for high-resistance dusts than with normal or low-resistance dusts.
One obvious solution to the problem is to increase the conductivity of the dust which is processed. The art has recognized this and in many cases has provided for the introduction of moisture or for the humidification of the dust before the gas stream enters the assembly. Naturally, this procedure cannot be used in all cases and depends upon the nature of the gas stream, the nature of the dust and the parameters under which the system operates. However, it is important to improve the efficiency of dust-collecting assemblies for high-resistance dusts.